This invention relates to a vacuum interrupter comprising a pair of composite electrodes disposed in a vacuum vessel, each having a main electrode and a coil electrode serving as a parallel magnetic field generator which generates a magnetic field in a direction parallel to an arc developing across the main electrodes when the composite electrodes separates from each other.
Generally, the vacuum interrupter has a cylindrical vacuum vessel and a pair of main electrodes which are associated with each other and mounted to respective electrical conductor rods extending from interior to exterior of the vacuum vessel. The paired main electrodes are normally closed for current conduction but in the event of failure occurring in an external circuit, they are opened or separated to protect a loaded electrical apparatus from being damaged. Upon the separation, an arc develops across the paired main electrodes, which is desired to be extinguished as rapidly as possible.
An approach to the extinction of arc is disclosed in, for example, U.S. Pat. No. 3,946,179 (corresponding to U.K. Pat. No. 1,478,702) or U.S. Pat. No. 4,196,327 (corresponding to U.K. Pat. No. 1,573,350), according to which in a vacuum interrupter a magnetic field is applied in parallel to an arc so as to disperse it into innumerable, fine fiber-like segmental arcings. More particularly, the former U.S. patent proposes a pair of composite electrodes each mounted to the tip of an electrical conductor rod and having a main electrode and a coil electrode behind the main electrode. The coil electrode includes a plurality of arm conductors for passing currents in directions which are radial of the central conductor rod, and a plurality of circumferential conductors for changing the radial currents in the arm conductors into looping currents which are concentric with the conductor rod so as to generate parallel magnetic fields of the same polarity in an axial direction of the conductor rod. The paired composite electrodes are superposed or mated so that magnetic fields due to radial currents, coming into and out of the central conductor rod, are cancelled. More specifically, radial currents in one composite electrode have the same magnitude as but opposite direction to those of radial currents in the other composite electrode and hence magnetic fields generated at the arm conductors of the paired composite electrodes are cancelled. Consequently, the generation of magnetic fields by the coil electrodes is due to looping currents in the circumferential conductors which flow in the same circumferential direction in the paired composite electrodes. The parallel magnetic fields generated by the coil electrodes of the respective composite electrodes, having the same polarity and being applied in the axial direction, are concentrated to a central portion of the main electrode and conductor rod of the respective composite electrodes. Accordingly, magnetic flux density is high at the central portion and decreases toward the circumferential portion. With this construction, when the main electrodes are separated in response to a peak value of an accident current, a highly intensive parallel magnetic field will act on an arc. For example, when the accident current is 63 KA, the magnetic flux density at the central portion amounts up to 1000 T. Under the application of such a highly intensive magnetic field, however, the arc, which would be spread rapidly from the central to circumferential portions of the main electrode under the application of a suitably intensive parallel magnetic field, cannot any more spread to the circumferential portion but undergoes degeneracy at the central portion. As a result, the central portion will be fused terribly, giving rise to failure of interruption and consequent impairment of interruption performance.